It is well known that fluid flow may be regulated electronically by employing an electronically controlled fluid regulator for adjusting the pressure of the fluid upstream of a flow restrictor in response to a control signal generated by a pressure sensor positioned upstream of the flow restictor. In the automotive field, such fluid regulation might be employed in the intake manifold or fluid distribution system of a gasoline engine. In the field of gas chromatography, the gas chromatograph (GC) detectors require accurately regulated fluid supplies, the distribution of which is performed by a pneumatic manifold.
A gas chromatographic apparatus, well known in the prior art, is shown in FIG. 1. The chromatograph 10 is arranged in a forward pressure regulated design suitable for direct injection. The detector 30 can be any of the GC detectors known in the art Typically, the detector 30 determines the magnitude of the physicochemical property over time. The performance of many chromatographic detectors is dependent upon the flow rate or pressure of the support fluid employed. By modifying the pressure or flow rate of the detector support fluids, one can for example, optimize detector sensitivity in an flame photometric detector (FPD) or flame ionization detector (FID), or to minimize solvent quenching of an nitrogen phosphorous detector (NPD) bead.
As well known in the prior art, and as illustrated in FIG. 1, a typical detector pneumatic manifold supplies three fluid sources 13a, 13b, 13c to three separate valves 14a, 14b, 14c. The valves serve to control both the pressure and the flow rate of the support fluid components. Flow through flow restrictors 15a, 15b, 15c provide back pressure such that sensors 16a, 16b, 16c can generate stable electronic signals in relation to the pressures or the flow rates of the component fluids. Pressure signals are provided to a processor 40, which in turn provides control signals to the valves 14a, 14b, 14c to regulate the pressure of the component fluids.
The processor 40 can maintain the pressure at some desired level by generating control signals directing the operation of the valves 14a-c. Since the generated control signals are in a digital form, they are converted to analog form by a digital to analog converter and appropriately amplified by an amplifier prior to transmission to valves 14a-c. The fluid pressures as sensed by the sensors 16a-c are provided to the processor 40 by first converting the analog signals generated by the pressure sensors 16a-c from an analog to digital signal by an A/D converter. The digital signals generated by the converter are supplied to the processor 40.
Unfortunately, the flow rate of a fluid through the flow restrictors 15a-c is unstable and varies with flow restrictor construction, the type of fluid flowing through the flow restrictor, the temperature of the fluid (essentially the manifold temperature) and the pressure of the fluid both upstream and downstream from the flow restrictor. Additionally, the pressure sensors 16a-c are sensitive to variations in temperature which can lead to errors in flow regulation. There exists a need for more stable detector fluid flows and reduced manifold temperature variations to provide better chromatographic area repeatability.
One method for eliminating temperature sensitivity is to enclose the flow sensing and controlling devices in a controlled heated zone constructed with thermally insulating material. Temperature sensors and heaters inside the heated zone provide feedback to maintain the flow restrictor and pressure sensor temperatures constant and thereby remove temperature as an error-producing variable.
Unfortunately, the incorporation of a heated zone increases manufacturing costs related to instrument calibration and components. Additionally, instrument reliability is reduced as the components required to regulate a heated zone are more likely to fail with continual operation at manifold temperatures higher than ambient. Furthermore, a heated zone requires a long start-up time to stabilize prior to instrument operation.
Another technique for correcting the inaccuracy of the pressure flow relationship due to flow restrictor variation and temperature dependencies is to perform extensive, multi-point calibrations at a very large number of different operating temperatures and pressures. The results of such calibrations are saved in an EEPROM and employed for adjusting the feedback signal to the control valve. The calibration points relate pressure sensor signals outputs, ambient pressure signal outputs, and a temperature signal to fluid flow rate through the flow restrictor. A separate set of calibrations is required for each fluid and flow restrictor combination. Unfortunately, the cost of such calibrations make this technique commercially unreasonable.
A need exists for a pneumatic manifold design which automatically compensates fluid flows for ambient temperature and pressure changes without the use of a heated zone.